Chemical vapor deposition (CVD) is defined as the formation of a non-volatile solid film on a substrate by the reaction of vapor phase reactants. The gaseous reactants are introduced into a reactor vessel, and decompose and react at a heated surface of the substrate to form the desired film. Chemical vapor deposition is often the preferred process for depositing thin films on substrates such as semiconductor wafers, principally because of its ability to provide highly conformal layers even within deep contacts and other openings.
CVD methods have been utilized in the past to deposit titanium nitride (TiN) films. In microelectric devices, TiN films can be used as low resistance contacts and as diffusion barriers in interconnect metallization schemes.
The method of depositing TiN by CVD utilizing the reaction of TiCl.sub.4 and N.sub.2 or NH.sub.3 has been known for many years. However, this method has several severe drawbacks. Foremost among these is the high (600-700.degree. C.) temperature required for deposition of pure films. The incorporation of unbonded chlorine is inversely related to the deposition temperature. These two traits make TiCl.sub.4 based CVD TiN wholly inappropriate for use in aluminum based IC processing at the via levels. However, the use of TiCl.sub.4 for the specialized application of contact metallization has been recently proven. As a further reason for the inappropriateness of this method to IC processing, the reaction is prone to depositing salts/adducts on chamber walls which in turn lead to particulate generation and cleaning concerns.
Two metal-organic compounds have received a significant amount of study recently; tetrakis(dimethlyamido)titanium (TDMAT), and tetrakis(diethylamido)titanium (TDEAT). These compounds have the ability to deposit TiN by CVD when co-reacted with a nitrogen source or from pure pyrolysis. But under certain conditions, including pyrolysis and very low co-reactant concentrations, the film will contain significant amounts of carbon and will be porous leading to moisture/oxygen absorption upon exposure to the atmosphere. Due to the differences in reaction kinetics, the resultant films deposited from these two metal-organic compounds are distinctly and significantly different. For similar processing conditions, the TDEAT reaction always produces a film that is much lower (orders of magnitude) in bulk resistivity, significantly lower in carbon content, and much more stable in terms of atmospheric exposure (changes in resistivity). Furthermore, films deposited from TDEAT have significantly better step coverage when processed within the same process regime.
While TDEAT does have the above advantages, it has certain disadvantages that result in processing difficulties. First, at a given temperature, the vapor pressure of TDEAT is about two orders of magnitude lower than TDMAT; this can lead to difficulties in delivering a usable amount of TDEAT to the reactor vessel. Second, the deposition rate of a CVD TiN film derived from TDEAT is about one-fourth the rate of TDMAT; this can lead to an unacceptably low throughput.
Therefore, a need exists for an improved process for manufacturing TiN films that reduces these difficulties.